1. Field of the Invention
This invention relates generally to the automated control of plants and factories and, more particularly, to the control unit that provides the interface with the gauges, valves, etc. that monitor and control the activity of the plants and factories.
2. Description of the Related Art
In the related art, the use of redundancy in associated processing and control circuits to prevent interruption of processes is well known. Using this technique, when an active member of the redundant circuits fails, then the here-to-fore inactive member becomes the active member and the processing can continue without interruption.
The modern process control systems frequently include a central control and data processing system which can be referred to as the plant control network. Exchanging signal groups with the plant control network are subsidiary units, which can be referred to as process control units. The process control units receive control instructions from the plant control network and forward status information to the plant control network. Based on the control instructions received from the plant control network and status signals from sensor devices monitoring the process activity, the process control unit can provide control signals to the devices actually used to determine the activity of the process. The process control unit can make local decisions, based on status signals, for changes required in the process and can transmit signals to the process control devices in an appropriate format, thereby relieving the plant control network of these activities.
When the process control unit includes redundant controllers, the instructions stored in each controller must be identical and, in addition, must be stored chronologically in the same order in each controller. Otherwise, when control changed from the primary or active controller to the redundant secondary or standby (inactive) controller, inappropriate decisions with potentially serious consequences can result.
A need has therefore been felt for apparatus and technique for insuring that the stored data groups used for process control in redundant controllers are identical and have been entered in storage units of the redundant controllers in the same order.